Honestly, I just think it is because of the problems it proposes upon the economy (negative possibilites). Think of it this way. A VERY large population of the United States smokes marijuana (obviously behind closed doors). These people pay a large amount of money for marijuana because it is "scarce" so to speak; not in the sense that it is hard to find (even though in a lot of places it is) but in the sense that not everyone is growing it because they are scared of consequences. You take away those consequences and marijuana becomes as cheap as cigarettes due to the fact that every mom and pop are growing it. Aside from the fact that no money would be put into the economy in the long run due to home-growers, the economy goes through a change due to this.
This change is: Marijuana becomes legalized and every home-grower begins to sell at their normal rate to everyone they can. The economy is now stable due to this new cash flow. Now that there is new cash flow and a better economy, there is more money to be put into the pockets of each american. These americans, even grocery store baggers, have an increased wage. Lets say minimum wage goes up from $5 to $6 or even $7. These americans now have more money to spend on, amongst other things, marijuana (which is now taking part in fueling the economy). Again, stability. However, as everyone tries growing their own weed, and previous home-growers are able to better fund their setups and can grow in whole rooms as opposed to stealth boxes: more marijuana is being provided to the community. Therefore, due to supply and legalization, we have more smokers arise, realizing they can put their "oh my gosh MJ is bad" ideals to bed. Still, the economy is stable. Everyone is happy, the economy is doing great, and quite possibly the minimum wage, amongst other forms of payment to the american population, are increased.
You're probably saying, "This guy is nuts. He's just contradicting himself!" However, time changes all things. Production soon outweighs consumption and/or the readily available MJ is now lowered in value due to "Well, I can get the same amount for half the price at Bob's house. Why would I buy from Jim?" "Well, Jim is now selling at 1/2 price also, but if you buy 10% more, he'll sell it for a little less, still."
And now begins the problems of legalizing marijuana. Like I previously said, marijuana becomes near the price of cigarettes (of course it could still be twice as much, but that's a lot less than what we pay.) Market giants begin creating marijuana factories and hell, even Wally world sells an abundance of it. So, you now have the entire nation getting this minimum wage of $8 or $9 an hour, along with the other forms of payments, but marijuana is drastically cheaper. Now the government expenditures slowly begin to outweigh their gains due to the loss of marijuana funding. The US economy begins borrowing more than ever before and our population in all aspects (medical, military, housing, etc.) begins to dwindle as we drop from a "world power" to a "world power supporter."
Just a thought of a stoner.